A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
To manufacture devices using lithographic techniques, it is necessary to form a number of patterned layers in accurate relative positions. To this end, it is common to form a plurality of markers on the substrate at the same time as the patterns required to form the desired devices. There are two main types of such markers: alignment markers and overlay markers. Alignment markers are usually formed in a single exposure step or process layer and are designed so that their position can be accurately measured with a suitable sensor device. They are used to define a reference position for printing one or more subsequent layers. Overlay markers have two components that are formed in different layers. The overlay marker is designed so that any difference in the position of the two layers, commonly referred to as an overlay error or simply overlay, is easily measurable. It is often not possible to accurately measure the exact position of an overlay marker.